Operations Management Chapter 17 – Maintenance and Reliability

Strategic Importance of Maintenance and Reliability Failure has far reaching effects on a firm’s Operation Reputation Profitability Dissatisfied customers Idle employees Profits becoming losses Reduced value of investment in plant and equipment

Maintenance and Reliability The objective of maintenance and reliability is to maintain the capability of the system while controlling costs Maintenance is all activities involved in keeping a system’s equipment in working order Reliability is the probability that a machine will function properly for a specified time

Important Tactics Reliability Maintenance Improving individual components Providing redundancy Maintenance Implementing or improving preventive maintenance Increasing repair capability or speed

Maintenance Strategy Employee Involvement Information sharing Skill training Reward system Employee empowerment Results Reduced inventory Improved quality Improved capacity Reputation for quality Continuous improvement Reduced variability Maintenance and Reliability Procedures Clean and lubricate Monitor and adjust Make minor repair Keep computerized records Figure 17.1

Reliability Improving individual components Rs = R1 x R2 x R3 x … x Rn where R1 = reliability of component 1 R2 = reliability of component 2 and so on

Overall System Reliability Reliability of the system (percent) Average reliability of each component (percent) | | | | | | | | | 100 99 98 97 96 100 – 80 – 60 – 40 – 20 – 0 – n = 10 n = 1 n = 50 n = 100 n = 200 n = 300 n = 400 Figure 17.2

Reliability Example R1 .90 R2 .80 R3 .99 Rs Reliability of the process is Rs = R1 x R2 x R3 = .90 x .80 x .99 = .713 or 71.3%

Product Failure Rate (FR) Basic unit of measure for reliability FR(%) = x 100% Number of failures Number of units tested FR(N) = Number of failures Number of unit-hours of operating time Mean time between failures MTBF = 1 FR(N)

Failure Rate Example 2 FR(%) = (100%) = 10% 20 2 20 air conditioning units designed for use in NASA space shuttles operated for 1,000 hours One failed after 200 hours and one after 600 hours FR(%) = (100%) = 10% 2 20 FR(N) = = .000106 failure/unit hr 2 20,000 - 1,200 MTBF = = 9,434 hrs 1 .000106

Failure Rate Example 2 FR(%) = (100%) = 10% Failure rate per trip 20 20 air conditioning units designed for use in NASA space shuttles operated for 1,000 hours One failed after 200 hours and one after 600 hours Failure rate per trip FR = FR(N)(24 hrs)(6 days/trip) FR = (.000106)(24)(6) FR = .153 failures per trip FR(%) = (100%) = 10% 2 20 FR(N) = = .000106 failure/unit hr 2 20,000 - 1,200 MTBF = = 9,434 hr 1 .000106

Providing Redundancy Provide backup components to increase reliability + x Probability of first component working Probability of needing second component Probability of second component working (.8) + x (1 - .8) = .8 + .16 = .96

Reliability has increased from .713 to .94 Redundancy Example A redundant process is installed to support the earlier example where Rs = .713 R1 0.90 R2 0.80 R3 0.99 Reliability has increased from .713 to .94 = [.9 + .9(1 - .9)] x [.8 + .8(1 - .8)] x .99 = [.9 + (.9)(.1)] x [.8 + (.8)(.2)] x .99 = .99 x .96 x .99 = .94

Maintenance Two types of maintenance Preventive maintenance – routine inspection and servicing to keep facilities in good repair Breakdown maintenance – emergency or priority repairs on failed equipment

Implementing Preventive Maintenance Need to know when a system requires service or is likely to fail High initial failure rates are known as infant mortality Once a product settles in, MTBF generally follows a normal distribution Good reporting and record keeping can aid the decision on when preventive maintenance should be performed

Computerized Maintenance System Data Files Personnel data with skills, wages, etc. Equipment file with parts list Maintenance and work order schedule Inventory of spare parts Repair history file Output Reports Inventory and purchasing reports Equipment parts list Equipment history reports Cost analysis (Actual vs. standard) Work orders Preventive maintenance Scheduled downtime Emergency maintenance Data entry Work requests Purchase requests Time reporting Contract work Figure 17.3

Maintenance Costs The traditional view attempted to balance preventive and breakdown maintenance costs Typically this approach failed to consider the true total cost of breakdowns Inventory Employee morale Schedule unreliability

cost maintenance policy) Maintenance Costs Costs Maintenance commitment Total costs Preventive maintenance costs Breakdown maintenance costs Optimal point (lowest cost maintenance policy) Traditional View Figure 17.4 (a)

cost maintenance policy) Maintenance Costs Full cost of breakdowns Costs Maintenance commitment Total costs Preventive maintenance costs Optimal point (lowest cost maintenance policy) Full Cost View Figure 17.4 (b)

Maintenance Cost Example Should the firm contract for maintenance on their printers? Number of Breakdowns Number of Months That Breakdowns Occurred 2 1 8 6 3 4 Total: 20 Average cost of breakdown = $300

Maintenance Cost Example Compute the expected number of breakdowns Number of Breakdowns Frequency 2/20 = .1 2 6/20 = .3 1 8/20 = .4 3 4/20 = .2 ∑ Number of breakdowns Expected number of breakdowns Corresponding frequency = x = (0)(.1) + (1)(.4) + (2)(.3) + (3)(.2) = 1.6 breakdowns per month

Maintenance Cost Example Compute the expected breakdown cost per month with no preventive maintenance Expected breakdown cost Expected number of breakdowns Cost per breakdown = x = (1.6)($300) = $480 per month

Maintenance Cost Example Compute the cost of preventive maintenance Preventive maintenance cost Cost of expected breakdowns if service contract signed Cost of service contract = + = (1 breakdown/month)($300) + $150/month = $450 per month Hire the service firm; it is less expensive

Increasing Repair Capabilities Well-trained personnel Adequate resources Ability to establish repair plan and priorities Ability and authority to do material planning Ability to identify the cause of breakdowns Ability to design ways to extend MTBF

How Maintenance is Performed Operator Maintenance department Manufacturer’s field service Depot service (return equipment) Preventive maintenance costs less and is faster the more we move to the left Competence is higher as we move to the right Figure 17.5

Total Productive Maintenance (TPM) Designing machines that are reliable, easy to operate, and easy to maintain Emphasizing total cost of ownership when purchasing machines, so that service and maintenance are included in the cost Developing preventive maintenance plans that utilize the best practices of operators, maintenance departments, and depot service Training workers to operate and maintain their own machines

Establishing Maintenance Policies Simulation Computer analysis of complex situations Model maintenance programs before they are implemented Physical models can also be used Expert systems Computers help users identify problems and select course of action